Benefits of System-Managed Storage

With SMS, you can define performance goals and data availability requirements, create model data definitions for typical data sets, and automate data backup. SMS can automatically assign, based on installation policy, those services and data definition attributes to data sets when they are created. IBM® storage management-related products determine data placement, manage data backup, control space usage, provide data security, and perform disaster backup and recovery.

The goals of system-managed storage are:

Improve the use of the storage media; for example, by reducing out-of-space abends and providing a way to set a free-space requirement.
Reduce the labor involved in storage management by centralizing control, automating tasks, and providing interactive or batch controls for storage administrators.
Reduce the user's need to be concerned with the physical details of performance, space, and device management. Users can focus on using information instead of managing data.

There are several benefits of system-managed storage.

Simplified Data Allocation

System-managed storage enables users to simplify their data allocations. For example, without using the Storage Management Subsystem, a z/OS® user would have to specify the unit and volume on which the system should allocate the data set. The user would also have to calculate the amount of space required for the data set in terms of tracks or cylinders. This means the user has to know the track size of the device which will contain the data set.

With system-managed storage, users can let the system select the specific unit and volume for the allocation. They can also specify size requirements in terms of megabytes (MB) or kilobytes (KB). This means the user does not need to know anything about the physical characteristics of the devices in the installation.

SMS is required if you want to allocate data sets in extended format, or specify compression or extended addressability. It is also required if you want to specify partial release, system-managed buffering, or a secondary volume allocation amount for VSAM data sets.

With DFSMS, you do not need model DSCBs for creating generation data sets.

Ensured Data Integrity on New Allocations

The system provides data integrity for newly allocated data sets that have not been written to. For these data sets, whether SMS managed or non-SMS managed, DFSMSdfp writes a physical end-of-file character at the beginning of the data set when space for the data set is initially allocated.

This makes it unnecessary to OPEN data sets for the sole purpose of writing an EOF and to avoid reading old data if the data set is read immediately after being allocated.

Improved Allocation Control

System-managed storage enables you to set a threshold for free space across a set of direct access storage device (DASD) volumes. During allocation of new data sets, the system prefers those volumes that are below the specified threshold. This allows existing data sets to be extended on the volumes that are above the threshold.

SMS reduces space-related abends on initial allocation or when extending to a new volume through the following:

Removing the DADSM "five extent limit"
Spreading the requested allocation space quantity over multiple volumes
Reducing the requested space quantity by a specified percentage

These do not apply while extending the data set on the same volume.

Volume selection techniques from the secondary list help to avoid problems, such as over-allocation of all new data sets on a newly added volume until it reaches a high threshold or until the available free space on the volume reaches the same level as other volumes in the storage group.

You can also set a threshold for scratch tape volumes in tape libraries, to ensure enough cartridges are available in the tape library for scratch mounts.

Improved Input/Output (I/O) Performance Management

System-managed storage enables you to improve DASD I/O performance across the installation and at the same time reduce the need for manual tuning by defining performance goals for each class of data. You can use cache statistics recorded in system management facilities (SMF) records to help evaluate performance. You can also improve sequential performance by using striped extended-format sequential or VSAM data sets. The DFSMS environment makes the most effective use of the caching abilities of the IBM 3990 Model 3 and Model 6 Storage Controls, as well as other models.

You can also use the DFSMS Optimizer feature to perform in-depth analysis of high I/O activity data sets, including recommendations for placement and simulations for cache and expanded storage.

For more information on the DFSMS Optimizer feature, see DFSMS Optimizer User's Guide and Reference.

Automated DASD Space Management

System-managed storage enables you to automatically reclaim space which is allocated to old and unused data sets or objects. You can define policies that determine how long an unused data set or object resides in primary storage (storage devices used for your active data). You can have the system remove obsolete data by migrating the data to other DASD, tape, or optical volumes, or you can have the system delete the data. You can also release allocated but unused space which is assigned to new and active data sets.

Tape Mount Management

System-managed storage lets you fully use the capacity of your tape cartridges and automate tape mounts. Using tape mount management (TMM) methodology, DFSMShsm can fill tapes to their capacity. With 3592 tape devices, media types such as IBM Enterprise Advanced Tape Cartridge (MEDIA11), recording modes such as EFMT4 and EEFMT4, and the improved data recording capability, you can increase the amount of data that can be written on a single tape cartridge.

Recommendation: With TMM, you must analyze tape mounts, modify ACS routines to redirect allocations intended for tape to a DASD pool, then migrate them to tape with the DFSMShsm interval migration. Alternatively, you can use the IBM Virtual Tape Server (VTS) to fill tape media, reduce tape mounts, and save system resources. For more information, see Using the Virtual Tape Server (VTS) to Optimize Tape Media.

System-Managed Tape

System-managed storage lets you exploit the device technology of new devices without having to change the JCL UNIT parameter. In a multi-library environment, you can select the drive based on the library where the cartridge or volume resides. You can use the IBM TotalStorage™ Enterprise Automated Tape Library (3494 or 3495) to automatically mount tape volumes and manage the inventory in an automated tape library. Similar function is available in a system-managed manual tape library. If you are not using SMS for tape management, you can still access the IBM TotalStorage Enterprise Automated Tape Library (3494 or 3495) using Basic Tape Library Storage (BTLS) software.

You can use the Virtual Tape Server (VTS), with or without the tape mount management methodology, to optimize your use of tape media. You might still need to use tape mount methodology for small tape data sets, but VTS improves your use of tape media and reduces tape mounts. Use VTS for volumes that don't require removal from the library for offsite storage. VTS integrates the advanced technology provided by the IBM 3590 tape drives, IBM fault-tolerant RAID disk storage, a RISC-based controller, and the IBM 3494 tape library.

Automated Storage Management for Object Data

System-managed storage enables you to fully use tape, DASD and optical cartridge capacity. Using an IBM 3995 Optical Library Dataserver, you can automatically mount optical volumes and manage the inventory in an automated optical library.

Related Reading: For more information about object data, object storage groups, and object backup backup storage groups, see z/OS DFSMS OAM Planning, Installation, and Storage Administration Guide for Object Support.

Improved Data Availability Management

With system-managed storage, you can provide different backup requirements to data residing on the same DASD volume. Thus, you do not have to treat all data on a single volume the same way.

You can use DFSMShsm to automatically back up the following:

CICS® databases
DATABASE 2^™ (DB2®^™) databases
Partitioned data sets extended (PDSEs)
Physical sequential data sets
Partitioned data sets
Virtual storage access method (VSAM) data sets
Direct access data sets
Hierarchical file system (HFS) data sets
Tip: For HFS data sets, the Tivoli® Storage Manager/UNIX System Services client backs up the files and directories inside the HFS data set and DFSMShsm backs up the data set itself. You can set up your procedures to back up the whole file system less frequently using guaranteed backup frequency, and then use the Tivoli Storage Manager client to back up data within the file system more often. This reduces the total recovery time since it uses the high bandwidth of DFSMShsm to perform backups and recoveries in case the file system becomes inaccessible.

You can also back up other types of data and use point-in-time copy to maintain access to critical data sets while they are being backed up. Concurrent copy, virtual concurrent copy, SnapShot, and FlashCopy®^™, along with backup-while-open, have an added advantage in that it avoids invalidating a backup of a CICS VSAM KSDS due to a control area or control interval split.

To backup and recover critical applications requiring concurrent action, such as for disaster recovery, you can create a logical grouping of data sets known as an aggregate group. You define an aggregate group by selecting a management class and specifying backup attributes (such as type of storage medium, retention period, or destination) which all data sets in the group share. DFSMShsm uses the aggregate backup and recovery support (ABARS) to manage the aggregate group backup process. You can also use ABARS to transfer applications between sites.

You can use the same management class attributes for multiple aggregate groups whose backup copies have the same management needs. These backup attributes are used to manage backup copies and also to create the proper environment for backed-up data sets during recovery. During aggregate recovery, data sets backed up as migrated data sets can be returned to the same level as when backed up, or they can all be recovered to ML1 DASD or ML2 tape. All data sets backed up from user volumes are returned to user volumes.

Simplified Movement of Data to Different Device Types

With system-managed storage, you can move data to new volumes without requiring users to update their job control language (JCL). Because users in a DFSMS environment do not need to specify the unit and volume which contains their data, it does not matter to them if their data resides on a specific volume or device type. This lets you easily replace old devices with new ones.

You can also use system-determined block sizes to automatically reblock physical sequential and partitioned data sets that can be reblocked.